Carbon-Coated Silicon Particle Powder as the Anode Material for Lithium Ion Batteries and Method of Making the Same

ABSTRACT

A process for the production of coated silicon/carbon particles comprising:
         providing a carbon residue forming material;   providing silicon particles;   coating said silicon particles with said carbon residue forming material to form coated silicon particles;   providing particles of a carbonaceous material;   coating said particles of carbonaceous material with said carbon residue forming material to form coated carbonaceous particles;   embedding said coated silicon particles onto said coated carbonaceous particles to form silicon/carbon composite particles;   coating said silicon/carbon composite particles with said carbon residue forming material to form coated silicon/carbon composite particles; and   stabilizing the coated composite particles by subjecting said coated composite particles to an oxidation reaction.       

     The coated composite particles will have a substantially smooth coating. The particles may be coated with multiple layers of carbon residue forming material/

TECHNICAL FIELD OF THE INVENTION

The present invention relates to silicon/carbon composite materials thatare useful as electrode active materials in batteries. Moreparticularly, the present invention relates to carbon-coated siliconparticles that find particular use as electrode materials, as well asmethods for the manufacture of said carbon-coated silicon particles.

BACKGROUND OF THE INVENTION

Synthetic graphites are widely used as standard negative electrodematerials in lithium ion batteries. Other carbonaceous materials arealso widely used in such batteries due to their efficiency andreasonable cost. Lithium ion batteries are primarily used as powersources in portable electronic devices. Compared to other classes ofrechargeable batteries such as nickel-cadmium and nickel-metal hydridestorage cells, lithium ion cells have become increasingly popular due torelatively high storage capacity and rechargeability.

Due to increased storage capacity per unit mass or unit volume oversimilarly rated nickel-cadmium and nickel-metal hydride storage cells,the smaller space requirements of lithium ion cells allow production ofcells that meet specific storage and delivery requirements.Consequently, lithium ion cells are popularly used in a growing numberof devices, such as digital cameras, digital video recorders, computers,etc., where compact size is particularly desirable from a utilitystandpoint.

Nonetheless, rechargeable lithium ion storage cells are not withoutdeficiencies. These deficiencies may be minimized with the use ofimproved materials of construction. Commercial lithium ion batterieswhich use synthetic graphite electrodes are expensive to produce andhave low relatively lithium capacities. Additionally, graphite productscurrently used in lithium ion electrodes are near their theoreticallimits for energy storage (372 mAhr/g). Accordingly, there is a need inthe art for improved electrode materials that reduce the cost ofrechargeable lithium batteries and provide improved operatingcharacteristics, such as higher energy density, greater reversiblecapacity and greater initial charge efficiency. There also exists a needfor improved methods for the manufacture of such electrode materials.

Silicon has been investigated as an anode material for lithium ionbatteries because silicon can alloy with a relatively large amount oflithium, providing greater storage capacity. In fact, silicon has atheoretical lithium capacity of more than ten times that of graphite.However, pure silicon is a poor electrode material because its unit cellvolume can increase to more than 300% when lithiated. This volumeexpansion during cycling destroys the mechanical integrity of theelectrode and leads to a rapid capacity loss during battery cycling.Although silicon can hold more lithium than carbon, when lithium isintroduced to silicon, the silicon disintegrates and results in lesselectrical contact which ultimately results in decreased ability torecharge the storage cell.

Continuous research efforts in solving silicon volume expansion problemshave yielded limited results. Silicon/carbon composite particles orpowders have good cycle life compared to mechanical mixtures of carbonand silicon powders made by milling or other mechanical methods. Thinfilm silicon-coated carbon particles or carbon-coated silicon powdersare potential replacements for graphite powders as the anode materialfor next generation lithium ion batteries. However, chemical vapordeposition methods typically used to apply silicon coatings or carboncoatings have intrinsic shortcomings that include slow deposition ratesand/or expensive precursors for deposition. Vapor deposited siliconfilms may be extremely expensive relative to the cost of bulk siliconpowders. Therefore, another method of manufacturing coated siliconparticles is needed.

SUMMARY OF THE INVENTION

The present invention provides processes for the manufacture of thesilicon/carbon composite materials. The silicon/carbon compositematerials comprise coated silicon particles that are combined withcoated carbon particles; wherein the resulting silicon/carbon compositeparticle is further coated with a layer of oxidized, carbonresidue-forming material. These carbon-coated silicon/carbon compositeparticles are useful in the manufacture of electrodes in electricalstorage cells, particularly in rechargeable lithium ion electricalstorage cells.

The compositions of the invention provide high capacity and highefficiency carbon-coated silicon/carbon composite particles that can bederived from a wide variety of carbon sources. In a further aspect ofthe invention, the silicon/carbon composite particle may be coated withmultiple layers of carbon residue forming material. In a still furtheraspect of the invention, the coating layer(s) of the composite particlemay be optionally carbonized.

The compositions of the present invention provide carbon-coatedsilicon/carbon composite particles with substantially smooth coatings.Additionally, the compositions feature good powder flowability, which isparticularly beneficial during the handling or manufacturing stepsnecessary to form these materials into useful electrodes or into otherarticles not specifically described herein.

In further aspects of the invention there are provided methods for themanufacture of such carbon-coated silicon/carbon composite particles.The carbon-coated powders prepared in accordance with the invention notonly increase charge efficiency but also provide excellentprocessability for electrode fabrication. In a yet further aspect of theinvention there are provided methods for the manufacture of electricalstorage cells, particularly rechargeable batteries that include saidcarbon-coated composite particles. A still further aspect of theinvention relates to the use of said carbon-coated composite particlesin electrical storage cells, particularly in rechargeable batteries.

These and other aspects and features of the invention will becomeapparent from the following description of the invention and preferredembodiments thereof.

DESCRIPTION OF DRAWINGS

FIG. 1 shows a schematic view of a composite carbon-silicon particleaccording to the present invention.

FIG. 2 shows a comparison of charge and discharge potential profiles onthe first cycle for different low cut-off potentials for silicon/carboncomposite particles and uncoated silicon particles.

FIG. 3 shows a scanning electron microscopy image of silicon/carboncomposite particles as prepared in Example 2.

FIG. 4 shows the discharge capacity and charge efficiency within thecharge/discharge potential window between 0.09 and 1.5 volts during thefirst 5 cycles for the silicon/carbon composite particles produced inExample 2.

FIG. 5 shows the capacity and columbic efficiency duringcharge/discharge cycles between 0.09 and 1.5 volts for the compositesilicon/carbon particles as prepared in Example 3.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

The present invention provides processes for the manufacture ofsilicon/carbon composite particles, which particles exhibit improvedoperating characteristics when used as electrodes in electrical storagecells, particularly in rechargeable electrical storage cells. Generally,the process contemplates combining coated fine silicon powders withcoated carbonaceous particles to form a silicon/carbon compositeparticle and further coating the composite particle with a layer orlayers of carbon residue-forming material.

More specifically, particles of a carbonaceous material substrate arecoated with a fusible, carbon residue-forming material. Particles offine silicon powders, which have been coated with a fusible, carbonresidue-forming material, are embedded onto the coated carbonaceousparticle to form a composite particle of silicon and carbonaceousmaterials. The silicon/carbon composite particle is further providedwith at least one coating of a fusible, carbon residue-forming material.The coated silicon/carbon composite particle is thereafter stabilized bysubjecting said coated composite particle to an oxidation reaction usingan oxidizing agent. The stabilized coated composite particle isthereafter carbonized.

While it is possible to embed uncoated silicon particles onto a coatedcarbonaceous substrate material, it is preferable that the siliconparticles be coated prior to embedding the silicon onto the carbonaceoussubstrate material to achieve an enhancement in cycling ability andmechanical strength over that of composite particle comprising uncoatedsilicon powder.

The silicon/graphite composite particle may be further coated withadditional layer(s) of carbon residue-forming material followingstabilization or optional carbonization.

It is preferable to apply a coating onto the carbonaceous particle priorto applying the silicon particles. It is preferable to embed coatedsilicon particles onto the coated carbonaceous substrate. Alternatively,uncoated silicon particles may be embedded onto the coated carbonaceoussubstrate. Further, it is preferable to coat the silicon/carboncomposite particle to enhance the mechanical strength of the composite,resulting in longer lasting silicon composite electrodes. Preferably,the process provides carbon-coated silicon/carbon composite particleshaving substantially smooth coatings. Optionally, the compositeparticles may be coated repeatedly with carbon residue forming materialto further increase the mechanical strength of the particles.

In the preferred embodiment, particles of carbonaceous substratematerial are required for the practice of the invention. These may beobtained from a variety of sources, examples of which include petroleumand coal tar cokes, synthetic and natural graphite, or pitches as wellas other sources of carbonaceous materials that are known in themanufacture of prior art electrodes, although these sources are notelucidated here. Preferred sources of carbonaceous materials includecalcined or uncalcined petroleum cokes as well as synthetic graphite.Preferred sources of carbonaceous materials also include calcined oruncalcined, highly crystalline “needle” cokes. Particularly preferredsources of carbonaceous material include natural graphite and flakecoke. Thus, preferred carbonaceous materials are either graphiticmaterials or materials which form graphite on heating to graphitizationtemperatures of 2200° C. or higher.

Fine particles of such carbonaceous substrate material are convenientlyprovided by milling, crushing, grinding or by any other means that canbe used to provide a pulverant carbonaceous substrate material havingparticles of dimensions that are suitable for use in the formation ofelectrodes. Although the principles of the present invention arebelieved to be applicable to carbonaceous substrate particles of varyingsizes and particle size distributions, preferred carbonaceous substrateparticles having average particle sizes up to about 50 μm, morepreferably from about 1 to about 30 μm.

Particles of silicon are required for the practice of the invention;such particles may be used alone or in conjunction with the carbonaceoussubstrate material. The purity of the silicon may be of ordinaryindustrial strength, i.e., 97-98 wt. %. Although the principles of thepresent invention are believed to be applicable to silicon particles ofvarying sizes and particle size distributions, preferred siliconparticles having average particle sizes up to about 50 μm, morepreferably from about 0.03 to about 20 μm.

According to steps in the inventive process, the silicon particles,carbonaceous substrate particles and silicon/carbon composite particlesare provided with a fusible, carbon residue-forming material as acoating material. Preferred for use as coating materials are carbonresidue-forming materials that are capable of being reacted with anoxidizing agent. Preferred compounds include those with a high meltingpoint and a high carbon yield after thermal decomposition. Exemplaryuseful coating materials include heavy aromatic residues from petroleum,chemical process pitches; lignin from pulp industry; phenolic resins;and carbohydrate materials such as sugars and polyacrylonitriles.Especially preferred for use as coating materials are petroleum and coaltar pitches and lignin that are readily available and have been observedto be effective as fusible, carbon residue-forming materials.

It is to be understood that the carbon residue-forming material providedas the coating for the carbonaceous, silicon or silicon/carbon compositeparticles, as the case may be, may be any material which, when oxidizedand then thermally decomposed in an inert atmosphere to a carbonizationtemperature of 850° C. or an even greater temperature forms a residuewhich is “substantially carbon”. It is to be understood that“substantially carbon” indicates that as the residue is at least 95% byweight carbon, it is also preferred that the carbon residue-formingmaterial form at least 10%, and preferably at least 40% and morepreferably at least 60% carbon residue on carbonization, based on theoriginal mass of the carbon residue-forming coating for the carbonaceoussubstrate, silicon or silicon/carbon composite particle.

It should be understood that the coatings used for one type of particlemay vary significantly from the coatings used for another type ofparticle. By way of non-limiting examples, the carbon residue formingmaterial provided as a coating for the carbonaceous substrate particlesmay be composed of a completely different carbon residue formingmaterial as that provided as a coating for the silicon particles or forthat provided as a coating for composite particles. Further, subsequentcoatings provided for composite particles may be composed of carbonresidue forming materials that differ from coatings applied to thecarbonaceous or silicon particles, or from the previous coatings on thecomposite particles.

Any organic compound that can be oxidized and then thermally decomposedto yield carbon residue can be used as the coating material. However, incoating processes in which the organic compounds are dissolved insolvent, aromatics compounds that include various molecular weights arepreferred because of the mutual dissolution of the compound with thesolvent. Preferred compounds include those with a high melting point anda high carbon yield after thermal decomposition (e.g., petroleum andcoal tar pitches).

Any useful technique for coating the carbonaceous, silicon or compositeparticles may be used. By way of non-limiting examples, usefultechniques include the steps of: liquefying the carbon residue-formingmaterial by a means such as melting or forming a solution with asuitable solvent combined with a coating step such as spraying theliquefied carbon residue-forming material onto the subject particle, ordipping the particle in the liquefied carbon residue-forming materialand subsequently drying out any solvent.

A particularly useful method of forming a uniform coating of a carbonresidue-forming material by precipitating the material onto the surfaceof the carbonaceous, silicon or silicon/carbon composite particles isprovided according to the following process. First, a concentratedsolution of the carbon residue-forming material in a suitable solvent isformed. The solution of carbon residue-forming material is prepared bycombining the carbon residue-forming material with a solvent or acombination of solvents. The solvent should be compatible with thecarbon residue-forming material and should dissolve all or a substantialportion of the coating material. Solvents include pure organic compoundsor a mixture of different solvents. The choice of solvent(s) depends onthe particular coating material used.

Suitable solvents for dissolving the carbon residue-forming materialinclude, e.g., benzene, toluene, xylene, quinoline, tetrahydrofuran,naphthalene, acetone, cyclohexane, and tetrahydronaphthalene (sold byDupont under the trademark Tetralin), ether, water andmethyl-pyrrolidinone, etc. When petroleum or coal tar pitch is used asthe carbon residue-forming material or coating material, e.g., solventssuch as toluene, xylene, quinoline, tetrahydrofuran, Tetralin, ornaphthalene are preferred. The ratio of the solvent(s) to the carbonresidue-forming material for the carbonaceous, silicon or compositeparticle in the solution and the temperature of the solution iscontrolled so that the carbon residue-forming material completely oralmost completely dissolves into the solvent. Typically, the solvent tocarbon residue-forming material ratio is less than 2, and preferablyabout 1 or less, and the carbon residue-forming material is dissolved inthe solvent at a temperature that is below the boiling point of thesolvent.

Concentrated solutions wherein the solvent-to-solute ratio is less than2:1 are commonly known as flux solutions. Many pitch-type materials formconcentrated flux solutions wherein the pitch is highly soluble whenmixed with the solvent at solvent-to-pitch ratios of 0.5 to 2.0.Dilution of these flux mixtures with the same solvent or a solvent inwhich the carbon residue-forming material is less soluble results inpartial precipitation of the carbon residue-forming coating material.When this dilution and precipitation occurs in the presence of asuspension of carbonaceous, silicon or composite particles, theparticles act as nucleating sites for the precipitation. The result isan especially uniform coating of the carbon residue material on theparticles.

The coating layer of the subject particle, whether carbonaceoussubstrate, silicon, or silicon/carbon composite, can be applied bymixing the particles into a solution of carbon residue-forming materialdirectly. When the particles are added to the solution of carbonresidue-forming material directly, additional solvent(s) is generallyadded to the resulting mixture to effect partial precipitation of thecarbon residue-forming material. The additional solvent(s) can be thesame as or different than the solvent(s) used to prepare the solution ofthe carbon residue-forming materials.

An alternative method to precipitation would require a suspension ofcarbonaceous substrate, silicon or silicon/carbon composite particles beprepared by homogeneously mixing the particles in the same solvent usedto form the solution of carbon residue-forming material, in acombination of solvent(s) or in a different solvent to a desiredtemperature, preferably below the boiling point of the solvent(s). Thesuspension of the target particles is then combined with the solution ofcarbon residue-forming material causing a certain portion of the carbonresidue-forming material to deposit substantially uniformly on thesurface of the particles.

The total amount and morphology of the carbon residue-forming materialthat precipitates onto the surface of a particle depends on the portionof the carbon residue-forming material that precipitates out from thesolution, which in turn depends on the difference in the solubility ofthe carbon residue-forming material in the initial solution and in thefinal solution. When the carbon residue-forming material is a pitch,wide ranges of molecular weight species are typically present. Oneskilled in the art would recognize that partial precipitation of such amaterial would fractionate the material such that the precipitate wouldbe relatively high molecular weight and have a high melting point, andthe remaining solubles would be relatively low molecular weight and havea low melting point compared to the original pitch.

The solubility of the carbon residue-forming material in a given solventor solvent mixture depends on a variety of factors including, forexample, concentration, temperature, and pressure. As stated earlier,dilution of concentrated flux solutions causes solubility to decreasesince the solubility of the carbon residue-forming material in anorganic solvent increases with temperature, precipitation of the coatingis further enhanced by starting the process at an elevated temperatureand gradually lowering the temperature during the coating process. Thecarbon residue-forming material can be deposited at either ambient orreduced pressure and at a temperature of about −5° C. to about 400° C.By adjusting the total ratio of the solvent to the carbonresidue-forming material and the solution temperature, the total amountand hardness of the precipitated carbon residue-forming material on thecarbonaceous, silicon or composite particles can be controlled.

The suspension of carbonaceous substrate, silicon or silicon/carboncomposite particles in the final diluted solution of carbonresidue-forming material generally has a ratio of solvent to carbonresidue-forming material of greater than about 2; and preferably greaterthan about 4. It would be understood by one skilled in the art that thespecific solvent to carbon residue-forming pitch ratio at the conclusionof the coating process depends on the carbon residue-forming materialand solvent selected for the process. On one hand, it is desirable touse as little solvent as possible because of the cost of solvent, whileon the other hand, enough solvent is required so that the particles canbe dispersed in the solvent.

Upon completion of the precipitation step, the coated particles areseparated from the mixture of solvent, particles, and carbonresidue-forming material using conventional methods, such as, forexample, centrifugal separation, or filtration. The particles areoptionally washed with solvent to remove residual pitch (or other carbonforming residue forming material) solution and dried using conventionalmethods.

According to an inventive step of the process, the silicon/carboncomposite particle is produced by co-precipitating pitch onto a mixtureof uncoated fine silicon powder particles and coated, relatively coarsecarbonaceous particles simultaneously, thereby effectively embeddingsilicon particles onto the coating layer of the relatively largecarbonaceous substrate particles. The resulting silicon/carbon compositeparticle is thereafter coated with pitch.

Alternatively, the silicon/carbon composite particle may be produced byseparately coating silicon particles and carbonaceous substrateparticles with pitch in separate containers; thereafter the coatedparticles are mixed together in a solution of pitch and solvent to embedthe coated silicon particle onto the coated carbonaceous substrateparticle.

According to a further step of the invention process, the coating layerof the silicon, carbon and silicon/carbon composite particles arerendered partly or completely infusible, preferably by oxidativestabilization. The coating of the particles are stabilized by subjectingsaid particles to an oxidation reaction using an oxidizing agent underappropriate reaction conditions. Generally, only mild-to-moderatereaction conditions are required. Typically, contacting the coatedparticles with an oxidizing agent at mild conditions and activating theoxidizing agent at elevated conditions satisfactorily perform theoxidation reaction. Contact with the oxidizing agent can occur atambient temperatures (approximately 20° C.) or at moderately elevatedtemperatures (up to approximately 400° C.). Activation of the oxidizingagent would typically occur at moderately elevated temperatures up to400° C. Preferably the temperature of the oxidation reaction ismaintained below the instantaneous melting point of the coating materialso as to insure that melting point of the coating material is notexceeded during the oxidation reaction.

According to a further step of the inventive process, the stabilizedcoated silicon, carbonaceous substrate particles or silicon/carboncomposite particles may be optionally carbonized. The degree to whichthe surface of the coating is rendered infusible by stabilization isdependent upon the type of pitch used as well as the solvents orcombination of solvents used. Further, if multiple layers of coating aredesired, it is preferable to apply additional layers of coatingfollowing stabilization or carbonization. The final coating on acomposite particle with multiple coatings is preferably carbonized.

The stabilization step of the current invention is carried out to renderthe surface of the coating layer infusible to the subsequentcarbonization. Oxidative stabilization allows the smooth surfaceproduced in the coating process to be preserved in the coated compositeparticles of the instant invention, as the oxidative stabilizationrenders the surface of the coating infusible to the subsequentprocessing steps.

Heat treatment of the stabilized coated particles is desirably conductedin a controlled manner in order to minimize fusion of the particles. Oneskilled in the art will recognize that highly stabilized, infusible,coated particles can be heated relatively aggressively and quicklyduring carbonization. In contrast, relatively mildly stabilized coatedparticles require slower heating in order to avoid excessive melting ofthe coating and fusion of the particles. Use of a fluidized bed duringstabilization and heat treatment is especially beneficial in preventingclumping and fusion of the coated particles.

With regard to the temperature required to insure carbonization forcoated particles, desirably this is achieved by raising the temperaturein a controlled manner from a starting temperature, usually ambienttemperature, to the final carbonization temperature which falls withinthe above-identified range of about 400° C. to about 1500° C.,preferably within the range of about 800° C. to about 1300° C., and morepreferably within the range of about 900° C. and 1200° C.

With regard to the atmospheric conditions for the carbonization processfor the stabilized coated particles, the atmosphere may be ambient airup to about 850° C. but an inert atmosphere is preferred at temperaturesabove about 400° C. Ambient air is an acceptable atmosphere when theoxygen is largely displaced during heating or during heating undervacuum. Suitable inert atmospheres include nitrogen, argon, helium,etc., which are non-reactive with the heated coated particles.

It is understood that during the heating of the coated particles,particular attention must be paid to ensure that neither thetemperatures attained during this heating process, nor the rate of thetemperature rise during any part of the heating process be such that theinstantaneous melting point of the coating on the particles is exceeded.More simply stated, the thermal degradation of the coating is to beeffected by a controlled temperature rise wherein the processtemperature is maintained at or below the instantaneous melting point ofthe coating where said melting point is generally increasing with timeduring the process. In view of this requirement, preferred heatingprocesses are those that exhibit slower rates of temperature rise.

The most preferred aspects of the invention result in the provision of asmooth coating upon the silicon/carbon composite particles. Preferablythe stabilization of the coating of the silicon/carbon compositeparticle is followed by controlled heating of the coated stabilizedsilicon/carbon composite particles so as to effect carbonization of thecoated particles with little or no clumping or self-adhesion of theindividual particles. The desired results are coated particles withlittle or no broken fracture surfaces of the type which arecharacteristically formed when the separate particles fuse and must becrushed or broken apart in order to provide a free flowing powder. Suchfracture surfaces are desirably minimized or avoided, as they arebelieved to contribute to low electrochemical efficiency when theparticles are used as an anode material in rechargeable electricalstorage cells, particularly in rechargeable lithium ion batteries.

According to a particularly preferred embodiment of the inventiveprocess taught herein, the carbon residue forming material is providedin a fluid form. It has been observed by the inventors that when thecarbon residue forming material is precipitated from a liquid, a smoothcoating forms at the interface of the individual carbonaceous particlesand the surrounding liquid. A smooth coating is retained whensubsequently carbonized.

Although less advantageous, when the carbon residue-forming coating issupplied as a solid, it is desirably fused on the surface of theparticles in order to form a smooth coating thereon. Especiallypreferred embodiments of the present invention produce a free-flowingpowder of coated particles after the carbonization, which particlesexhibit little or no fusion among the particles, but can generally bebroken into a free-flowing powder by simple mechanical agitation, suchas by use of a stirring rod, or by rubbing between the thumb andforefinger. Where some fusion may have occurred between particles, andmechanical agitation is used to separate these particles which mayresult in the formation of new fracture surfaces, in the preferredembodiments of the invention these fracture surfaces do not comprisemore than 10%, preferably no more than 2% of the total surface area ofthe particles. Such are considered as being substantially smoothcoatings.

A preferred aspect of the present invention is in the pitch coatingprocess, or carbon residue-forming material coating process. Thiscoating process provides uniform carbon residue-forming coating onparticles regardless of particle size. The coating can be accomplishedin a number of ways but it is especially advantageous to precipitate thecoating material in the presence of a suspension of the targetedparticles, whether silicon, carbonaceous substrate material orsilicon/carbon composite particles. This coating method yields a uniformcoating of controlled composition and produces a loose particle powder,so that the pitch-coated particles do not agglomerate and no furthermilling process is required in the subsequent process steps.

Another preferred aspect of the present invention is in an oxidationreaction that is carried out on the coated particles prior tocarbonization of the coating. The oxidation reaction is believed toprovide certain technical benefits. First, it is believed that thereacted coated particles are relatively infusible following oxidation,which is particularly desirable in view of subsequent process steps, andsubsequent handling of the particles. Second, it is believed that thereacted coated particles are endowed with a surface which yields highefficiency when used as an electrode, particularly when the reactedcoated particles are used in an anode material in a rechargeable storagecell, particularly in a rechargeable lithium ion cell.

A further aspect of the invention contemplates the use of coated siliconor coated silicon/carbon composite particles in electrodes, particularlyanodes, of electrical storage cells, particularly in rechargeablebatteries. According to this aspect of the invention, there iscontemplated a method for the manufacture of an electrical storage cellwhich comprises the steps of: incorporating into an anode of theelectrical storage cell silicon materials comprising silicon/carboncomposite particles having a coating layer formed of an oxidized, carbonresidue forming material.

According to this aspect of the invention, the coated silicon/carboncomposite particles produced from the processes described above areformed using the conventional techniques into electrodes, particularlyanodes. While not described with particularity herein, it iscontemplated that known-art manufacturing techniques for the assemblageof such electrodes, as well as known-art devices which facilitate in theformation of such electrodes can be used. A particular advantage whichis obtained by the use of the coated particles taught herein lies in thefact that due to their coating, they rarely fuse together thus resultingin a flowable powder.

Aspects of the present invention, including certain preferredembodiments are described in the following Examples.

EXAMPLE 1 Material Preparation

The silicon powder used in this example had an average particle size of5 μm (from Johnson Matthey Company). The pitch used for the coatinglayer was a petroleum pitch from Conoco, Inc. that was approximately 27%insoluble in xylene. The procedure for coating pitch on silicon powderis as follows. First, 20 grams of the silicon powder was mixed withabout 100 ml of xylene so that silicon particles were uniformlydispersed in xylene in a glass flask. Concurrently, 14 grams of thepitch was mixed with an equal amount of xylene in another flask so thatthe pitch was completely dissolved in xylene. Both the solutions wereheated to approximately 110° C. and the pitch solution was added intothe silicon solution while being continuously mixed. The resultingsolution was then heated to 140° C. and continuously stirred for about15 minutes. The solution was removed from the heater and the solutiongradually cooled to ambient temperature (˜25° C.). While the solutionswere mixed and cooled, the insoluble pitch precipitated out of solutionand coated uniformly onto the silicon particles. The resulting solidparticles in the solution are pitch-coated silicon powder. The powderwas then separated from the liquid by filtration and washed with 50 mlof xylene.

The pitch-coated silicon powders were then dried under vacuum at ˜100°C. The total weight of the dried powder was about 23.8 g, resulting in a16 wt % pitch coating on the silicon. The powders were then transferredinto a tube furnace and heated at 1° C./minute to 300° C. and heatedfurther for 10 hours at 300° C. under a reduced air pressure (typically˜−22″ Hg). During such heat treatment (stabilization), the weight of thepitch on silicon particles increased by about 5%. Followingstabilization, the powders were heated at 5° C./minute to a temperaturehigher than 1150° C. in nitrogen gas for 2 hours. Typically, the weightof the stabilized pitch decreased by about 25% during carbonization.Based on the amount of initial pitch prior to stabilization, the overallweight of the pitch decreased by about 20% or about 80% of the pitchremains as carbon coating after carbonization.

The resulting powder was then evaluated as the anode material for alithium-ion battery, as described below in the section “Evaluation ofElectrical Capacity”. FIG. 2 shows a comparison of the potentialprofiles during the first cycle charge and discharge for differentcut-off potentials. For comparison, the potential profiles of a mechanicmixture of plain silicon and graphite powders were also shown in thefigure. In this figure, the y-axis is the electrical potential of thesilicon electrode versus lithium metal during the charging anddischarging, the x-axis represents the charge stored into and removedfrom the electrode based on unit weight of the composite material. Theelectrical potential of the material is an indicator of the saturationlevel of lithium alloying; the lower the potential, the closer thematerial is to saturation. It can be seen that the ratio of coulombicefficiency is fairly high (>90%) for the composite carbon/siliconparticles, whereas it is very low (<30%) for the mechanic mixture ofgraphite and plain silicon. In addition, the capacity as defined in thenext section is very large for the carbon-coated silicon powder.

Evaluation of Electrical Capacity

The electrical reversible capacity and the coulombic efficiency of thepowder particles according to Examples 1-3 as well as ComparativeExamples were evaluated by the following techniques.

A uniform slurry was formed by thoroughly mixing powder (5 grams) with3.82 grams of a solution containing 0.382 grams of polyvinylidenefluoride (PVDF, ex. Aldrich Chemical Co., Inc.), 3.44 g of1-methyl-pyrrolidinone (NMP, ex. Aldrich Chemical Co., Inc.), and 0.082grams of acetylene black (having an effective surface area of 80 m²/g,ex. Alfa Aesar). The slurry was then manually cast utilizing a doctorblade to form a thin film having a loading of about 6 mg/cm² onto therough side of an electrodeposited copper foil (10 μm, ex. Fuduka MetalFoil & Powder Co., Ltd.). The cast film was then dried on a hot plate atapprox. 100° C. and pressed to a desired density (approx. 1.4 g/cm²)with a roll press. A disc having an area of 1.6 cm² was then punched outfrom the film and weighed to determine the exact amount of the mass onthe copper foil. Subsequently this disc was further dried under vacuumat a temperature of 80° C. for approximately 15 minutes and transferredinto a sealed box without exposing the disc to ambient air. The sealedbox was filled with ultra-pure argon gas having oxygen and moisturelevels of less than 1 ppm.

Subsequently the disc was cast as the positive electrode in themanufacture of a standard coin cell (2025 size) which was subsequentlyused as the test cell. The other electrode of the test cell was a foilof pure lithium (100 μm, ex. Alfa Aesar). A two layer separator was usedin the test cell: a glass mat (GF/B Glass Microfibre Filter, WhatmanInternational Ltd.) was used as the first layer on the compositeCarbon/silicon powder and a porous polypropylene film (available asCelgard® 2300, ex. Celgard Inc.) was used as the second layer on thelithium foil. The electrolyte of the test cell was a 1 M LiPF₆ inethylene carbonate (EC)/diethyl carbonate (DEC)/dimethyl carbonate (DMC)solvent mixture (40/30/30) (purchased from EM Industrial). Test cellswere produced utilizing the component described above according toconventional techniques, although the samples of powder particles werevaried to ensure that at least one sample coin cell was producedincorporating a powder particle sample according to either one of thedemonstrative examples, or according to one of the comparative examples.These powders were tested as the anode material in a coin cellconfiguration of carbon/separator/lithium metal at room temperature(˜25° C.). Two or three cells were made for each sample; the reportedcharge capacity and charge efficiency were the average value of thecells.

The capacity and charging efficiency of a specific powder particlesample was determined according to the following protocol. Utilizing astandard electrochemical test station (Model BT-2043, Arbin InstrumentCorp.), an assembled test cell was first discharged (equivalentlyalloyed with lithium) at 0.5 mA (approx. 52 mA/g) to a given voltage onthe first cycle. Thereafter, the assembled test cell was charged(de-alloyed) at 0.5 mA to 1.5 volts during which time the charge passedduring charging was used to calculate the specific capacity of thecomposite powder, while the ratio of the total charge passed duringcharging to the total charge passed during discharging was used todetermine the charge efficiency.

EXAMPLE 2

Twenty grams of a natural flake graphite powder (average particle size 5μm from China) were coated with 10 wt % petroleum pitch according to theprocedure as described in Example 1. The coated graphite powder wasstabilized, carbonized, and graphitized at 3000° C. in argon.Concurrently, a silicon powder (average particle size 2 μm, purchasedfrom Johnson Matthey company) was coated with 10 wt % pitch as describedin Example 1, stabilized, and carbonized at 1050° C. A mixture of thecoated natural graphite powder and the coated silicon powder werecombined in the proportion of 6 parts coated graphite and 4 parts ofcoated silicon powder and coated with 15 wt % solution of the same pitchusing the same method. After stabilization in air, the resultingcomposite powder was carbonized at 1050° C. in nitrogen atmosphere. Theresulting graphite/silicon/carbon composite particle powder hasmorphology as shown in FIG. 3. It can be seen that small siliconparticles are embedded in the carbon coating on large graphiteparticles, a structure similarly illustrated in FIG. 1.

The composite powder was then evaluated as the anode material for alithium-ion battery, as described above in the section entitled“Evaluation of Electrical Capacity”. The cycling potential window wasbetween 0.09 and 1.5 volts. The results are shown in FIG. 4. It can benoted that the material has a capacity of about 850 mAh/g and is fairlyreversible from cycle to cycle.

EXAMPLE 3

Twenty grams of a natural flake graphite powder (average particle size 5μm from China) were coated with 7 wt % petroleum pitch according to theprocedure as described in example 1. The coated graphite powder wasstabilized and carbonized at 1200° C. The coated graphite powder wasmixed with the coated silicon powder as described in Example 2, in thesame proportions. The mixture was then coated with 15 wt % pitch asdescribed in Example 1 and stabilized. Subsequently, the resultingcomposite particle powder was coated again with 10 wt % pitch,stabilized, and carbonized at 1050° C. in nitrogen atmosphere. Thematerial was evaluated as the anode material for Li-ion batteries in thesame manner as described previously. The capacity and efficiency of thismaterial are shown in FIG. 5 for the first five cycles. A significantincrease in the rechargeability of the silicon powder is displayed.

COMPARATIVE EXAMPLE

To compare the carbon-coated silicon powder with uncoated silicon powderat the same carbon coating level, electrodes were made by adding 20%graphite to uncoated silicon and 7% of the same graphite to thecarbon-coated silicon. The graphite used was natural graphite basedcomposite graphite powder.

FIG. 2 shows the charge and discharge cell voltage profiles for thecarbon-coated silicon and uncoated silicon powders. It should be notedthat “charging” means that the lithium is being electrochemicallyinserted into the electrode and “discharging” denotes that the lithiumis being removed from the electrode. The charge and discharge capacitywas calculated based on total electrode material except for the bindingmaterial. As shown in the figure, the cell voltage rapidly drops to thelow cut-off voltage on the charging and the discharged capacity and theefficiency are very small for the silicon/graphite mixture electrode.

1-36. (canceled)
 37. Coated silicon/carbon composite particlescomprising a core of silicon and carbonaceous particles wherein each ofthe silicon particles and carbonaceous particles are each coated with acarbon residue forming material, combined together and further coatedwith a layer of carbon residue forming material.
 38. The coatedcomposite particles according to claim 37, wherein the compositeparticles comprise a pulvurent carbonaceous material selected from thegroup consisting of petroleum pitches, calcined petroleum cokes,uncalcined petroleum cokes, highly crystalline cokes, coal tar cokes,synthetic graphites, natural graphites, soft carbons derived fromorganic polymers, and soft carbons derived from natural polymers. 39.The coated carbonaceous particles according to claim 37, wherein thecomposite particles are a pulvurent carbonaceous material selected fromthe group consisting of calcined petroleum cokes, uncalcined petroleumcokes, highly crystalline cokes, synthetic graphites, and naturalgraphites.
 40. The coated carbonaceous particles of claim 37 wherein thecoating layer is graphitic.
 41. A method for the production of a Li-ionbattery wherein the coated carbonaceous particles of claim 37 are usedas the anode material, and wherein such Li-ion battery exhibits a firstcycle charge efficiency greater than 90% at the cut-off potential of 1volt versus Li when tested with electrolyte containing no propylenecarbonate solvent.
 42. An electrical storage cell comprising the coatedcarbonaceous particles of claim
 37. 43. An electrical storage cellaccording to claim 42, wherein the electrical storage cell is arechargeable electrical storage cell.
 44. A method for the manufactureof an electrical storage cell which comprises incorporating the coatedcomposite particles of claim 37 into an anode of the electrical storagecell. 45-52. (canceled)
 53. A method for the manufacture of anelectrical storage cell, wherein the method comprises incorporating intoan anode of the electrical storage cell coated, silicon/carbon compositematerials comprising coated silicon particles and coated carbonaceousparticles having a coating layer formed of an oxidized, carbon residueforming material. 54-64. (canceled)